This invention relates to a telecommunication system including a plurality of terminal sets, a plurality of subscriber's line circuits, a plurality of subscriber's lines connecting the terminal sets with the subscriber's line circuits corresponding thereto, and a ringing signal generator for generating a ringing signal which is supplied to a particular one of the subscriber's lines upon reception of a call signal destined to the particular subscriber's line, and more particularly, to a ringing signal control circuit in the telecommunication system for controlling supply of the ringing signal to the particular subscriber's line.
Generally speaking, a telecommunication network system comprises exchanging stations such as exchanging office stations at nodes in a public telephone network and private exchanging stations or key telephone apparatus at network terminals. Each of these stations accommodates a plurality of subscriber's lines which extend to terminal sets such as telephone sets, facsimile sets, or the like, and station lines which extend to other stations. These lines will be referred to as subscriber's lines.
Each station includes subscriber's line circuits which are connected to the subscriber's lines, respectively, and a ringing signal control circuit for supplying a ringing signal to a particular one of the subscriber's lines when a particular one of the subscriber's line circuits receives a call signal destined to the particular subscriber's line to produce a call detection signal.
A conventional ringing signal controlling circuit comprises a ringing signal generator which generates the ringing signal in response to the call detection signal from any one of the subscriber's line circuits. The ringing signal cyclically changes in its voltage level between a maximum and a minimum voltage level with a frequency.
A switch control circuit generally starts upon receipt of the call detection signal and generates and maintains the switching signal during a predetermined time period, for example, one second which is so called a make period. The make period is followed by a pause period of, for example, two seconds. The switching signal is repeatedly generated with the pause period between one and the next successive make periods.
A plurality of switching elements are provided to connect the subscriber's lines to the subscriber's line circuits, respectively, and to connect the subscriber's lines to the ringing signal generator, respectively. Each one of the switching elements has a first switching mode in absence of a switching signal and a second switching mode in presence of the switching signal.
Each of the switching elements connects the corresponding one of the subscriber's line circuits to the corresponding one of the subscriber's line and disconnects the ringing signal generator from the corresponding subscriber's line during the first switching mode. Each of the switching elements is responsive to the switching signal for connecting the ringing signal generator to the corresponding subscriber's line and disconnects the corresponding subscriber's line circuit from the corresponding subscriber's line during the second switching mode.
Thus, when the particular subscriber's line circuit detects the call signal destined to the particular subscriber's line, the ringing signal is repeatedly supplied to the particular subscriber's line with the pause period between one make period and the next succeeding make period.
When a handset is lifted up at a particular one of the terminal sets connected to the particular subscriber's line, the particular subscriber's line circuit detects an off-hook signal from the particular terminal set through the particular subscriber's line during the pause period. Then, the particular subscriber's line circuit produces a stop signal. Responsive to the stop signal, the ringing singal generator and the switching control circuit stop operation.
In the conventional ringing signal control circuit, it will be noted that switching operation of the switching element is carried out in no relation to the voltage level of the ringing signal. However, the present inventor discovered a problem that a surge voltage was often caused on the subscriber's line when the subscriber's line was connected to, and disconnected from, the ringing signal generator. The surge voltage adversely affects, as a noise signal, to other subscriber's lines.
It was also found out that the surge voltage was not caused when the connection and disconnection of the subscriber's line to the ringing signal generator was carried out at a specific voltage level of the ringing signal. The specific voltage level is determined by properties of the subscriber's line and the terminal set connected to the subscriber's line.
Accordingly, the inventor previously proposed an improved ringing signal control circuit in a prior Japanese patent application No. 38657/1992 filed on Jan. 29, 1992 which was published on Aug. 13, 1993 under Publication No. JP-A-5 207136.
The proposed ringing signal control circuit is additionally provided with a reference voltage producing circuit for producing a reference voltage selected equal to the specific voltage level, and a voltage comparator for comparing the ringing signal with the reference voltage to produce a coincident signal when the ringing signal coincides with reference voltage in voltage level. After receiving the call detection signal, the switch control circuit produces the ringing signal in response to the coincident signal from the voltage comparator. Thereafter, when receiving the coincident signal after the make period, the switch control circuit stops the ringing signal. Thereafter, when receiving the coincident signal after the pause period, the switch control circuit again produces the ringing signal. These operations are repeated until receiving the stop signal from the subscriber's line circuit.
In the previously proposed ringing signal control circuit, the surge voltage is effectively suppressed on a particular one of the subscriber's lines which has the specific or optimum voltage level equal to the reference voltage level. However, the surge voltage is still caused on another subscriber's line having an optimum voltage level different from the reference voltage.
In order to resolve the problem, there is a method to provide a plurality of pairs of the reference voltage producing circuit and the voltage comparator for the subscriber's lines. However, the method makes the ringing signal control circuit expensive.